Generally, for the optical pick-up apparatus, a semiconductor laser such as LD (laser diode) is used as the light source. Because a diverging light flux projected from the semiconductor laser has an elliptical cross sectional shape (that is, a distribution of the light emitting strength is elliptical), it is necessary that the elliptical light flux from the semiconductor laser is converted to a circular light flux and a light utilization efficiency is increased.
As a beam shaping element (beam-shaper) to shape the sectional shape of the light flux from an elliptic to a circle, an optical element whose optical surface is an anamorphic surface or cylindrial surface, is well known. (For example, refer to Patent Documents 1 and 2).
(Patent Document 1) Tokkai No. 2003-178480
(Patent Document 2) Tokkai No. 2003-188452
(Patent Document 3) Tokkai No. 2002-323673
Recently, there are many cases where, in the optical pick-up apparatus, a light flux of short wavelength or a light flux of a large power is used. Therefore, there is a problem that a change of environmental temperature brings a change of performance (refractive index or shape) of an optical element composing an optical system or a change of a wavelength of a projected light flux from the semiconductor laser, and the astigmatism is generated. Accordingly, in order to suppress the change of the lens characteristic following a change of such a environmental temperature, generally, there are many cases where the optical element made of glass is used, and the optical element made of glass is adopted also for the beam-shaper disclosed in the Patent Documents 1 and 2. On the one hand, in the Patent Document 3, it is written that the beam-shaper made of plastic whose aberration change is small, is used.
When plastic is used, the performance change due to the temperature change is larger than glass. Also in Patent Document 3, a problem that the astigmatism is generated due to the temperature change by using the plastic shaping element is written, and it is written that the astigmatism is suppressed by the linear expansion of the plastic-made lens barrel provided between the light source and the beam-shaper.
However, in a method of the Document 3, because the linear expansion of the lens barrel provided between the light source and the beam-shaper is used for the astigmatism suppression, it is necessary to pay consideration for the selection of the material constituting the lens barrel for appropriately suppressing the astigmatism, or there are many limitations to the design work also for the thickness of the lens barrel itself or the length, and as a whole, there is a problem that the degree of freedom of the manufacture becomes narrow. Accordingly, specially, even when the element made of plastic is used, a result is brought about in which a merit of low cost•small size•light weight can not be fully exhibited.
Furthermore, in the case of the beam-shaper of the Document 3, because both of an incident surface and an outgoing surface are composed of a toric surface, the surface shape becomes complicated, as a result, a low cost element, apparatus can not be realized.
An object of the present invention considers the above problems and is to provide a beam-shaper, light source apparatus, and optical pick-up apparatus, which are excellent in a point of low cost, small size, light weight, by which, while various aberrations including the generation of the astigmatism following the change of environmental temperature are effectively suppressed, the divergent light flux whose sectional shape is almost circular, can be projected.